Cyclic aeration system for submerged membrane modules

ABSTRACT

An aeration system for a submerged membrane module has a set of aerators connected to an air blower, valves and a controller adapted to alternately provide a higher rate or air flow and a lower rate of air flow in repeated cycles. In an embodiment, the air blower, valves and controller, simultaneously provide the alternating air flow to two or more sets of aerators such that the total air flow is constant, allowing the blower to be operated at a constant speed. In another embodiment, the repeated cycles are of short duration. Transient flow conditions result in the tank water which helps avoid dead spaces and assists in agitating the membranes.

This is a continuation of U.S. Ser. No. 10/684,406, filed Oct. 15, 2003;which is a continuation of U.S. application Ser. No. 10/369,699, filedFeb. 21, 2003, issued as U.S. Pat. No. 6,706,189 on Mar. 16, 2004; whichis a continuation of U.S. application Ser. No. 09/814,737, filed Mar.23, 2001, issued as U.S. Pat. No. 6,550,747 on Apr. 22, 2003, which is acontinuation-in-part of U.S. application Ser. No. 09/488,359, filed Jan.19, 2000, issued as U.S. Pat. No. 6,245,239 on Jun. 12, 2001; which is acontinuation of international application number PCT/CA99/00940, filedOct. 7, 1999; which is an application claiming the benefit under 35 USC119(e) of provisional application No. 60/103,665, filed Oct. 9, 1998;and, 60/116,591, filed Jan. 20, 1999, both abandoned. The disclosures ofall patents and applications listed above, in their entirety, areincorporated herein by this reference to them.

FIELD OF THE INVENTION

This invention relates to filtering liquids and particularly to usingscouring air bubbles produced by an aeration system to clean or inhibitthe fouling of membranes in a submerged membrane filter.

BACKGROUND OF THE INVENTION

Submerged membranes are used to treat liquids containing solids toproduce a filtered liquid lean in solids and an unfiltered retentaterich in solids. For example, submerged membranes are used to withdrawsubstantially clean water from wastewater and to withdraw potable waterfrom water from a lake or reservoir.

The membranes are generally arranged in modules which comprise themembranes and headers attached to the membranes. The modules areimmersed in a tank of water containing solids. A transmembrane pressureis applied across the membrane walls which causes filtered water topermeate through the membrane walls. Solids are rejected by themembranes and remain in the tank water to be biologically or chemicallytreated or drained from the tank.

Air bubbles are introduced to the tank through aerators mounted belowthe membrane modules and connected by conduits to an air blower. The airbubbles rise to the surface of the tank water and create an air liftwhich recirculates tank water around the membrane module. When the rateof air flow is within an effective range, the rising bubbles and tankwater scour and agitate the membranes to inhibit solids in the tankwater from fouling the pores of the membranes. Further, there is also anoxygen transfer from the bubbles to the tank water which, in wastewaterapplications, provides oxygen for microorganism growth. The air blowergenerally runs continuously to minimize stress on the air blower motorsand to provide a constant supply of air for microorganism growth ifdesired.

With typical aeration systems, an operator increases the rate of airflow to the aerators if more cleaning is desired. This technique,however, stresses the membranes and air blower motors and increases theamount of energy used which significantly increases the operating costsof the process. Conversely, an operator typically decreases the rate ofair flow to the aerators if less cleaning is desired. With thistechnique, however, the rate of air flow is often below the effectiverange, which does not provide efficient cleaning. Alternately, someoperators reduce the average rate of air flow by providing airintermittently. This method allows for an air flow rate in the effectiverange but at the expense of the air blowers which wear rapidly whenturned off and on frequently. In many cases, the warranty on the airblower is voided by such intermittent operation.

Another concern with typical aeration systems is that they cause thetank water to move in a generally steady state recirculation pattern inthe tank. The recirculation pattern typically includes “dead zones”where tank water is not reached by the recirculating tank water andbubbles. The membranes in these dead zones, or the parts of themembranes in these dead zones, are not effectively cleaned and may beoperating in water having a higher concentration of solids than in thetank water generally. Accordingly, these membranes, or the affectedparts of these membranes, quickly foul with solids.

A related problem occurs in modules where hollow fibre membranes areinstalled with a small degree of slack to allow the membranes to moveand shake off or avoid trapping solids. The movement of tank water inthe tank encourages slackened membranes to assume a near steady stateposition, particularly near the ends of the membranes, which interfereswith the useful movement of the fibres.

Yet another concern with current aeration systems is that the aeratorsthemselves often foul over time. Even while the air supply is on, thelocal air pressure near the perimeter of the aerator holes is low andoften allows tank water to seep into the aerator. When aeration isstopped from time to time, for example for backwashing, cleaning orother maintenance procedures, more tank water may enter the aerationsystem. A portion of the tank water entering the aeration systemevaporates there, leaving deposits of solids in the aeration system. Inwastewater applications in particular, the deposited solids cansignificantly reduce the efficiency of the aeration system or cause anoperator to periodically shut down filtration to clean or replace theaerators.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a cyclic aerationsystem that may be used for aerating ultrafiltration and microfiltrationmembranes modules immersed in tank water in a tank. The cyclic aerationsystem uses a valve set and a valve set controller to connect an airsupply to a plurality of distinct branches of an air delivery network.The distinct branches of the air delivery network are in turn connectedto aerators located below the membrane modules. While the air supply isoperated to supply a steady initial flow of air, the valve set and valveset controller split and distribute the initial air flow between thedistinct branches of the air distribution system such that the air flowto each distinct branch alternates between a higher flow rate and alower flow rate in repeated cycles.

In one embodiment, the cyclic aeration system is used to provideintermittent aeration to membrane modules arranged in a plurality offiltration zones, each associated with a distinct branch of the airdelivery network. The cyclic aeration system is configured and operatedto provide aeration for a predetermined amount of time to eachfiltration zone in turn. In other embodiments, the cyclic aerationsystem is used to provide intense aeration to a group of membranemodules. In one such embodiment, the cyclic aeration system isconfigured and operated to provide air to a branch of the air deliverynetwork alternating between a higher flow rate and a lower flow rate incycles of 120 seconds or less. In another such embodiment, aeratorsassociated with a first branch of the air delivery network areinterspersed with aerators associated with a second branch of the airdelivery network. Air flow at a higher flow rate is alternated betweenthe first and second branches of the air delivery network in cycles of120 seconds or less.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be describedwith reference to the following figures.

FIG. 1A is a schematic drawing of a submerged membrane reactor.

FIGS. 1B, 1C and 1D are drawings of membrane modules according toembodiments of the present invention.

FIG. 2 is a plan view schematic of an aeration system according to anembodiment of the present invention.

FIG. 3 is a series of graphs showing the effect of operating anembodiment of the present invention.

FIGS. 4A, 4B, and 4C are schematic drawings of valve sets and valvecontrollers according to embodiments of the invention.

FIG. 5 is a plan view schematic of membrane modules and an aerationsystem according to an embodiment of the invention.

FIG. 6 is a plan view schematic of membrane modules and an aerationsystem according to another embodiment of the invention.

FIG. 7A is a plan view schematic of membrane modules and an aerationsystem according to another embodiment of the invention.

FIGS. 7B, 7C and 7D are elevational representations of membrane modulesand parts of an aeration system according to alternatives to theembodiment of FIG. 7A.

FIGS. 8A and 8B are elevational representations of membrane modules andparts of an aeration system according to an embodiment of the inventionunder the influence of a cyclic aeration system.

FIGS. 9A, 9B, 9C and 9D are drawings of aerators according to anembodiment of the invention.

FIGS. 10A, 10B and 10C are charts showing the results of tests performedon embodiments of the invention having two groups of aerators.

FIG. 11 is a chart showing the results of tests performed on embodimentsof the invention having a single group of aerators.

DETAILED DESCRIPTION OF EMBODIMENTS

General Description

Referring now to FIG. 1A, the general arrangement of a reactor 10 isshown. The description of the reactor 10 in this section appliesgenerally to various embodiments to be described below to the extentthat it is not inconsistent with the description of any particularembodiment.

The reactor 10 has a tank 12 which is initially filled with feed water14 through an inlet 16. The feed water 14 may contain microorganisms,suspended solids or other matter which will be collectively calledsolids. Once in the tank, the feed water 14 becomes tank water 18 whichmay have increased concentrations of the various solids, particularlywhere the reactor 10 is used to treat wastewater.

One or more membrane modules 20 are mounted in the tank and have one ormore headers 22 in fluid communication with a permeate side of one ormore membranes 6. The membranes 6 in the membrane modules 20 have a poresize in the microfiltration or ultrafiltration range, preferably between0.003 and 10 microns.

Membrane modules 20 are available in various sizes and configurationswith various header configurations. For example, the membranes 6 may behollow fibres potted in one or more headers 22 such that the lumens ofthe hollow fibres are in fluid communication with at least one header22. The headers 22 may be of any convenient shape but typically have arectangular or round face where they attach to the membranes 6.Alternatively, the membranes 6 may be flat sheets which are typicallyoriented vertically in a spaced apart pair with headers 22 on all foursides in fluid Communication with the resulting interior surface. Amembrane module 20 may have one or more microfiltration orultrafiltration membranes 6 and many membrane modules 20 may be joinedtogether to form larger membrane modules, or cassettes, but all suchconfigurations will be referred to as membrane modules 20.

FIGS. 1B, 1C and 1D illustrate preferred membrane modules 20 havingrectangular skeins 8. In each rectangular skein 8, hollow fibremembranes 23 are held between two opposed headers 22. The ends of eachmembrane 23 are surrounded by potting resin to produce a watertightconnection between the outside of the membrane 23 and the headers 22while keeping the lumens of the hollow fibre membranes 23 in fluidcommunication with at least one header 22. The rectangular skeins 8 maybe oriented in a horizontal plane (FIG. 1B), vertically (FIG. 1C) orhorizontally in a vertical plane (FIG. 1D). A plurality of rectangularskeins 8 are typically joined together in a membrane module 20.

Although a single row of hollow fibre membranes 23 is illustrated ineach rectangular skein 8, a typical rectangular skein 8 has a mass ofhollow fibre membranes 23 between 2 cm and 10 cm wide. The hollow fibremembranes 23 typically have an outside diameter between 0.4 mm and 4.0mm and are potted at a packing density between 10% and 40%. The hollowfibre membranes 23 are typically between 400 mm and 1,800 mm long andmounted with between 0.1% and 5% slack.

Referring again to FIG. 1A, the tank 12 is kept filled with tank water18 above the level of the membranes 6 in the membrane modules 20 duringpermeation. Filtered water called permeate 24 flows through the walls ofthe membranes 6 in the membrane modules 20 under the influence of atransmembrane pressure and collects at the headers 22 to be transportedto a permeate outlet 26 through a permeate line 28. The transmembranepressure is preferably created by a permeate pump 30 which creates apartial vacuum in a permeate line 28. The transmembrane pressure mayvary for different membranes and different applications, but istypically between 1 kPa and 150 kPa. Permeate 24 may also beperiodically flowed in a reverse direction through the membrane modules20 to assist in cleaning the membrane modules 20.

During permeation, the membranes 6 reject solids which remain in thetank water 18. These solids may be removed by a number of methodsincluding digestion by microorganisms if the reactor 10 is a bioreactoror draining the tank 12 periodically or by continuously removing aportion of the tank water 18, the latter two methods accomplished byopening a drain valve 32 in a drain conduit 34 at the bottom of thetank.

An aeration system 37 has one or more aerators 38 connected by an airdelivery system 40 and a distribution-manifold 51 to an air source 42,which is typically one or more air blowers, and produces bubbles 36 inthe tank water. The aerators 38 may be of various types includingdistinct aerators, such as cap aerators, or simply holes drilled inconduits attached to or part of the distribution manifold 51. Thebubbles 36 are preferably made of air but may be made of other gassessuch as oxygen or oxygen enriched air if required.

The aerators 38 are located generally below the membrane modules 20. Ifthe membrane modules 20 are made of rectangular skeins 8 having verticalhollow fibre membranes 23, the aerators 38 are preferably located toproduce bubbles near the edges of the lower header. With rectangularskeins 8 having hollow fibre membranes 23 in a vertical plane, theaerators 38 are preferably located to produce bubbles in a line directlybelow the vertical plane. With rectangular skeins 8 having hollow fibremembranes 23 in a horizontal plane, the aerators 38 are preferablylocated to produce bubbles evenly dispersed below the plane.

The bubbles 36 agitate the membranes 6 which inhibits their fouling orcleans them. In addition, the bubbles 36 also decrease the local densityof tank water 18 in or near the membrane modules 20 which creates anair-lift effect causing tank water 18 to flow upwards past the membranemodules 20. The air lift effect causes a recirculation pattern 46 inwhich the tank water 18 flows upwards through the membrane modules 20and then downwards along the sides or other parts of the tank. Thebubbles 36 typically burst at the surface and do not generally followthe tank water 18 through the downward flowing parts of therecirculation pattern 46. The tank water 18 may also flow according to,for example, movement from the inlet 16 to the drain conduit 34, butsuch flow does not override the flow produced by the bubbles 36.

The bubbles 36 have an average diameter between 0.1 and 50 mm.Individual large bubbles 36 are believed to be more effective incleaning or inhibiting fouling of the membranes 6, but smaller bubbles36 are more efficient in transferring oxygen to the tank water 18 andrequire less energy to produce per bubble 36. Bubbles 36 between 3 mmand 20 mm, and more preferably between 5 mm and 15 mm in diameter, aresuitable for use in many wastewater applications. Bubbles 36 in theranges described immediately above provide effective cleaning of themembranes 6 and acceptable transfer of oxygen to the tank water 18without causing excessive foaming of the tank water 18 at the surface ofthe tank 12. If the reactor 10 is used to create potable water or forother applications where oxygen transfer is not required, then bubblesbetween 5 mm and 25 mm are preferred.

The bubbles 36 may be larger than a hole in an aerator 38 where thebubble 36 is created according to known factors such as air pressure andflow rate and the depth of the aerators 38 below the surface of the tankwater 18. If the aerators 38 are located near the bottom of a large tank12, such as those used in municipal treatment works, an aerator 38 withholes of between 2 mm and 15 mm and preferably between 5 mm and 10 mmmight be used. The air pressure supplied (relative to atmosphericpressure) is typically determined by the head of water at the depth ofsubmergence of the aerators 38 (approximately 10 kPa per meter) plus anadditional pressure required to get the desired rate of air flow throughthe aerators 38. there is typically a pressure drop of between 5 mm and10 of water, and more typically between 10 mm and 50 mm of water acrossthe holes of the aerators 38. Parts of the aeration system 37 located ata distance below the bottom of the holes of the aerators 38 equal to thepressure drop are generally free of tank water when the air source 42 isoperating, although small amounts of tank water 18 may still seep intothe aeration system 37.

Cyclic Aeration System

Now referring to FIG. 2 a cyclic aeration system 237 is shown having anair supply 242 in fluid communication with a valve set 254, the valveset 254 controlled by a valve controller 256. The valve set 254 is influid communication with an air delivery network 240 having a pluralityof distinct branches each in fluid communication with distinct headers251 in fluid communication with conduit aerators 238. Other types ofaerators may also be used with suitable modifications to the headers 251or air delivery network, but conduit aerators 238 are preferred. Thethird branch of the air delivery network 240 and the third header 251are shown in dashed lines to indicate that the number of distinctbranches of the air delivery network 240 and headers 251 may be two ormore, but preferably net more than 15.

The air supply 242 is a source of pressurized air, typically one or moreair blowers, and provides a flow of a gas at an initial rate to thecyclic aeration system. The gas is most often air, but may also beoxygen, oxygen or ozone enriched air, or nitrogen in which cases the airsupply 242 will include oxygenation or ozonation equipment etc. inaddition to an air blower. In this document, however, the term “air”will be used to refer to any appropriate gas. The amount of air providedby the air supply 242 is best determined by summing the amount of airprovided to all conduit aerators 238 (to be described below) serviced bythe air supply 242. It is preferred that the air supply 242 supply aconstant amount of air over time.

The valve set 254 and valve controller 256 will be described in moredetail below. In general terms, however, the valve set 254 and valvecontroller 256 (a) split the air flow from the air supply 242 betweenthe branches of the air delivery network 240 such that, at a point intime, some of the branches receive air at a higher rate of air flow andsome of the branches receive air at a lower rate of air flow and (b)switch which branches of the air delivery network 240 receive the higherand lower rates of air flow in repeated cycles.

An example is illustrated in FIG. 3. In each of parts a), b), and c) ofFIG. 3, Rh indicates a higher rate of air flow; R1 indicates a lowerrate of air flow; and, the time from 0 to t3 indicates a cycle whichwould be repeated. The cycle is divided into three substantially equaltime periods, 0-t1; t1-t2; and, t2-t3. In each of these periods, onebranch of the air delivery system 240 and its associated manifold 251receive air at Rh while the others receive air at R1. Similarly, eachbranch of the air delivery system 240 and its associated manifold 251receives air at Rh for one third of the cycles and at R1 for two thirdsof the cycle.

Many of the valves sets 254 to be described below can be used to producesmooth variations in air flow rate to a manifold 251, but it ispreferred if the variation is fairly abrupt as suggested by FIG. 3. Theinventors have noticed that such an abrupt change produces a short burstof unusually large bubbles 36 which appear to have a significantcleaning or fouling inhibiting effect. The abrupt changes often alsoproduces a spike in air flow rate shortly after the transition from R1to Rh which produces a corresponding pressure surge. This pressure surgemust be kept within the design limits of the cyclic aeration system 237or appropriate blow off valves etc. provided.

The amount of air provided to a manifold 251 or branch of air deliverynetwork 240 is dependant on numerous factors but is preferably relatedto the superficial velocity of air flow for the conduit aerators 238services. The superficial velocity of air flow is defined as the rate ofair flow to the conduit aerators 238 at standard conditions (1atmosphere and 25 degrees Celsius) divided by the cross sectional areaof aeration. The cross sectional area of aeration is determined bymeasuring the area effectively aerated by the conduit aerators 238.Superficial velocities of air flow of between 0.013 m/s and 0.15 m/s arepreferred at the higher rate (Rh). Air blowers for use in drinking waterapplications may be sized towards the lower end of the range while airblowers used for waste water applications may be sized near the higherend of the range.

R1 is typically less than one half of Rh and is often an air offcondition with no flow. Within this range, the lower rate of air flow isinfluenced by the quality of the feed water 14. An air off condition isgenerally preferred, but with some feed water 14, the hollow fibremembranes 23 foul significantly even within a short period of aerationat the lower rate. In these cases, better results are obtained when thelower rate of air flow approaches one half of the higher rate.

Referring now to FIGS. 4A, 4B and 4C, alternative embodiments of thevalve set 254 and valve controller 256 are shown. In FIG. 4A, an airsupply 242 blows air into a three way valve 292, preferably a ballvalve, with its its two remaining orifices connected to two manifolds251. A three way valve controller 294 alternately opens an air pathwayto one of the manifolds 251 and then the other. Preferably there is aphase shift of 180 degrees so that the air pathway to one of themanifolds 251 opens while the airway to the other manifold 251 closes.The three way valve 292 may be mechanically operated by handle 296connected by connector 298 to a lever 299 on the three way valvecontroller 294 which is a drive unit turning at the required speed ofrotation of the lever 299. Preferably, however, the three way valvecontroller 294 is a microprocessor and servo or solenoid combinationwhich can be more easily configured to abruptly move the three way valve292.

In FIG. 4B, the air supply 242 blows air into a connector 261 whichsplits the air flow into a low flow line 262 and a high flow line 264. Avalve 266 in the low flow line 262 is adjusted so that flow in the lowflow line 262 is preferably less than one half of the flow in the highflow line 264. A controller 268, preferably a timer, a microprocessor orone or more motors with electrical or mechanical links to the valves tobe described next, controls a low valve 270, which may be a solenoidvalve or a 3 way ball valve, and a high valve 272, which may be asolenoid valve or a 3 way ball valve, so that for a first period of time(a first part of a cycle) air in the low flow line 262 flows to one ofthe manifolds 251 and air in the high flow line flows to the othermanifold 251. For a second period of time (a second part of a cycle),the low valve 270 and high valve 272 are controlled so that air in thelow flow line 262 flows to the a manifold 251 through cross conduit 274and air in the high flow line 264 flows to the other manifold 251through reverse conduit 276.

In FIG. 4C, air supply 242 blows air into a blower header 260 connectedby valves 262 to manifolds 251. Each valve 262 is controlled by a slavedevice 280, typically a solenoid or a servo motor. The slave devices areoperated by a microprocessor 282 programmed to open and close the valves262 in accordance with the system operation described in this sectionand the embodiments below.

Use of Cyclic Aeration to Provide Efficient Intermittent Aeration

Use of the cyclic aeration system 237 to provide efficient intermittentaeration will now be described with reference to the followingembodiment, it being understood that the invention is not limited to theembodiment. Referring to FIG. 5, an aeration system 237 is shown for usein providing intermittent aeration to six membrane modules 20 (shownwith dashed lines) in a filtration tank 412. The filtration tank 412 hassix filtration zones (also shown with dashed lines) corresponding to thesix membrane modules 20. Alternately, the filtration zones could beprovided in separate tanks with one or more membrane modules 20 in eachtank. The membrane modules 20 will be used to filter a relativelyfoulant free surface water such that intermittent aeration is suitable.

The air delivery network 240 has six distinct branches each connected toa header 251 in a filtration zone. Each header 251 is in turn connectedto conduit aerators 238 mounted generally below the membrane modules 20.The valve set 254 and valve controller 256 are configured and operatedto provide air from the air supply 242 to the air delivery network 240in a 7.5 minute cycle in which air at the higher rate is supplied forabout 75 seconds to each branch of the air delivery network 240 in turn.While a branch of the air delivery network 240 is not receiving air atthe higher rate, it receives air at the lower rate. Accordingly, eachheader 251 receives air at the higher rate for 75 seconds out of every7.5 minutes. Operation of the air supply 242, however, is constant andan air supply sized for one manifold 251 is used to service six suchmanifolds.

It is preferable if backwashing of the membrane modules 20 is alsoperformed on the membrane modules in turn such that backwashing of amembrane module 20 occurs while the membrane module 20 is being aerated.The membrane modules 20 can be backwashed most easily when each membranemodule 20 is serviced by its own permeate pump 30 and associatedbackwashing apparatus. In large municipal systems, for example, thepermeation and backwashing apparatus are typically limited to about 8 to11 ML/d capacity. Accordingly a medium size plant (ie. in the range of40 ML/d) will have several membrane modules 20 serviced by sets ofpermeation and backwashing apparatus which can be individuallycontrolled. In some plants, backwashing is performed on the membranemodules 20 in turn to produce an even supply of permeate 24 regardlessof aeration.

In a pilot study conducted with feed water having turbidity of 0.3 ntuand colour of 3.9 tcu, for example, the inventors were able to achieveacceptable sustained permeability of a membrane module using 75 secondsof aeration at a higher rate of 0.035 m/s superficial velocity every 15minutes and 15 seconds. For the remainder of the cycle there was noaeration. Each cycle involved 15 minutes of permeation through themembrane modules 20 and 15 seconds of backwashing. The 75 seconds ofaeration was timed so that there was 30 seconds of aeration before thebackpulse, aeration during the backpulse, and 30 seconds of aerationafter the backpulse. The test suggests that if cycled aeration is timedto coincide for each manifold 251 with the backwashing of the associatedmembrane module 20, then about 12 membrane modules 20 could be servicedby a single air supply 242 as part of the cyclic aeration system 237.

Use of Cyclic Aeration to Provide Intense Aeration

Use of the cyclic aeration system 237 to provide intense aeration willnow be described with reference to the following embodiment, it beingunderstood that the invention is not limited to the embodiment.Referring to FIG. 6, an aeration system 237 is shown for use inproviding aeration alternating between two sets of membrane modules 20(shown with dashed lines) in a filtration vessel 512. The filtrationvessel 512 has two filtration zones (also shown with dashed lines)corresponding to the two sets of membrane modules 20. Alternately, thefiltration zones could be provided in separate tanks with one or moremembrane modules 20 in each tank. The membrane modules 20 will be usedto filter a relatively foulant rich surface water or a wastewater suchthat intense aeration is suitable.

The air delivery network 240 has two distinct branches each connected toheaders 251 in a filtration zone. Each header 251 is in turn connectedto conduit aerators 238 mounted generally below the membrane modules 20.The valve set 254 and valve controller 256 are configured and operatedto provide air from the air supply 242 to the air delivery network 240in a short cycle in which air at the higher rate is supplied for onehalf of the cycle to each branch of the air delivery network 240. Whilea branch of the air delivery network 240 is not receiving air at thehigher rate, it receives air at the lower rate.

The preferred total cycle time may vary with the depth of the filtrationvessel 512, the design of the membrane modules 20, process parametersand the conditions of the feed water 14 to be treated, but preferably isat least 10 seconds (5 seconds at the full rate and 5 seconds at thereduced rate) where the filtration vessel 512 is a typical municipaltank between 1 m and 10 m deep. A cycle time of up to 120 seconds (60seconds at the full rate, 60 seconds at the reduced rate) may beeffective, but preferably the cycle time does not exceed 60 seconds (30seconds at the full rate, 30 seconds at the reduced rate) where thefiltration vessel 512 is a typical municipal tank.

The inventors believe that such rapid cycling creates transient flowwithin the tank water 18. In particular, an air lift effect is createdor strengthened when the rate of airflow changes from R1 to Rh causingthe tank water 18 to accelerate. Shortly afterwards, however, aerationand the air lift effect are sharply reduced causing the tank water 18 todecelerate. With very short cycles, the tank water 18 is accelerating ordecelerating for much of the cycle and is rarely in a steady state. Itis believed that formation of quiescent zones in the tank water 18 isinhibited and that beneficial movement of the hollow fibre membranes 23is enhanced. For example, horizontal hollow fibre membranes 23, as shownin the rectangular skeins 8 of FIGS. 1B and 1D, assume a generallyconcave downward shape under steady state aeration and experiencelimited movement at their ends. With cyclic aeration as described above,however, tension in the hollow fibre membranes 23 is released cyclicallyand, in some cases, local currents which flow downward may be createdfor brief periods of time. The ends of the horizontal hollow fibremembranes 23 experience more beneficial movement and foul less rapidly.Since the beneficial effects may be linked to creating transient flow,it is also believed that factors which effect acceleration of the watercolumn above a set of conduit aerators 238, such as tank depth orshrouding, could modify the preferred cycle times stated above.

Use of Cyclic Aeration to Promote Horizontal Flow

Use of the cyclic aeration system 237 to promote horizontal flow in thetank water 18 will now be described with reference to the followingembodiment, it being understood that the invention is not limited to theembodiment. Referring to FIG. 7 a, an aeration system 237 is shown foruse in aerating membrane modules 20 in a process tank 612. The membranemodules 20 will be used to filter a relatively foulant rich surfacewater or a wastewater such that intense aeration is suitable.

The air delivery network 240 has two distinct branches each connected totwo distinct headers 251, both in a single filtration zone. The headers251 will be referred to as header 251 a and 251 b where convenient todistinguish between them. Headers 251 are connected to conduit aerators238 such that the conduit aerators 238 attached to header 251 a areinterspersed with the conduit aerators 238 attached to header 251 b. Onesuch arrangement is shown in FIG. 7A in which header 251 a is connectedto conduit aerators 238 directly beneath the membrane modules 20 whileheader 251 b is connected to horizontally displaced conduit aerators 238located beneath and between the membrane modules 20. Referring now toFIGS. 7B, 7C and 7D, a set of variations of the embodiment of FIG. 7A isshown. In FIG. 7B, header 251 a and header 251 b are connected toalternating horizontally displaced conduit aerators 238 located beneaththe membrane modules 20. In FIG. 7C, header 251 a and header 251 b areconnected to alternating horizontally displaced conduit aerators 238located directly beneath alternating membrane modules 20. In FIG. 7C,header 251 a and header 251 b are connected to alternating horizontallydisplaced conduit aerators 238 located directly beneath and betweenalternating membrane modules 20. In each of these cases, the pattern maybe repeated where more membrane modules 20 are used.

Each of header 251 a and header 251 b are connected to a distinct branchof the air delivery network 240 in turn connected to a valve set 254.The valve set 254 and a valve controller 256 are configured and operatedto provide air from an air supply 242 to the air delivery network 240 ina short cycle in which air at a higher rate is supplied for one half ofthe cycle to each branch of the air delivery network 240. While a branchof the air delivery network 240 is not receiving air at the higher rate,it receives air at the lower rate. The lower flow rate is preferably onehalf or less of the higher flow rate and, where conditions allow it, thelower flow rate is preferably an air-off condition.

The total cycle time may vary with the depth of the filtration vessel512, the design of the membrane modules 20, process parameters and theconditions of the feed water 14 to be treated, but preferably is atleast 2 seconds (1 second at the full rate and 1 second at the reducedrate) and less than 120 seconds (60 seconds at the full rate, 60 secondsat the reduced rate) where the filtration vessel 412 is a typicalmunicipal tank between 1 m and 10 m deep. Preferably, however, the cycletime is between 20 seconds and 40 seconds in length. Short cycles of 10seconds or less may not be sufficient to establish regions of differentdensities in the tank water 18 in a deep tank 12 where such time isinsufficient to allow the bubbles 36 to rise through a significantdistance relative to the depth of the tank 12. Long cycles of 120seconds or more may result in parts of a membrane module 20 notreceiving bubbles 36 for extended periods of time which can result inrapid fouling. As discussed above, the beneficial effects of theinvention may be linked to creating transient flow and it is believedthat factors which effect acceleration of the water column above a setof conduit aerators 238, such as tank depth or shrouding, could modifythe preferred cycle times stated above.

In this embodiment, having the conduit aerators 238 connected to header251 a interspersed with the conduit aerators 238 attached to header 251b creates varying areas of higher and lower density in the tank water 18within a filtration zone. As described above, the inventors believe thatthese variations produce transient flow in the tank water 18. Where theeffective areas of aeration above conduit aerators 238 attached todistinct branches of the air delivery network 240 are sufficientlysmall, however, the inventors believe that appreciable transient flow iscreated in a horizontal direction between areas above conduit aerators238 attached to different branches of the air delivery network 240.Referring to FIGS. 7A, 7B, 7C, 7D the membrane modules 20 shown arepreferably of the size of one or two rectangular skeins 8.

As an example, in FIGS. 8A and 8B membrane modules 220 made ofrectangular skeins 8 with hollow fibre membranes 23 oriented verticallyaerated by a cyclic aeration system 237 with conduit aerators 238located relative to the membrane modules 220 as shown in FIG. 7D. InFIGS. 8A and 8B, the degree of slack of the hollow fibre membranes 23 ishighly exaggerated for easier illustration. Further, only two hollowfibre membranes 23 are illustrated for each vertical rectangular skein 8although, as discussed above, a rectangular skein 8 would actually beconstructed of many hollow fibre membranes 23.

With steady state aeration, it is difficult to encourage bubbles 36 topenetrate the vertical rectangular skeins 8. The natural tendency of thebubbles 36 is to go through the areas with lowest resistance such asaround the membrane modules 220 or through slots between the membranemodules 220 and the hollow fibre membranes 23 on the outer edge of thevertical rectangular skeins 8 may have significantly more contact withthe bubbles 36. Further, the upper 10-20% of the hollow fibre membranes23 is often forced into a tightly curved shape by the air lift effectand moves only very little. A smaller portion at the bottom of thehollow fibre membranes 23 may also be tightly curved by the currenttravelling around the lower header 22. In these tightly curved areas,the hollow fibre membranes 23 foul more rapidly.

With cyclic aeration, however, air at the higher rate is alternatedbetween header 251 a and header 251 b. When more air is supplied toheader 251 a, the hollow fibre membranes 23 assume an average shape asshown in FIG. 8A with a first local recirculation pattern 280 as shown.When more air is supplied to header 251 b, the hollow fibre membranes 23assume an average shape as shown in FIG. 8B with a second localrecirculation pattern 282 as shown. Under the influence of a cyclicaeration system 237, the hollow fibre membranes 23 alternate between thepositions shown in FIGS. 8A and 8B. Accordingly, the portion of thehollow fibre membranes 23 which moves only very little is decreased insize. The cycling also creates a reversing flow into and out of thevertical rectangular skeins 8 which the inventors believe encouragesbubbles 36 to penetrate deeper into the vertical rectangular skeins 8.

Conduit Aerators

Now referring to FIG. 9A, a conduit aerator 238 is shown. The conduitaerator 238 has an elongated hollow body 302 which is a circular pipehaving an internal diameter between 15 mm and 100 mm. A series of holes304 pierce the body 302 allowing air to flow out of the conduit aerator238 to create bubbles. The size, number and location of holes may varybut for a rectangular skein 8, for example, 2 holes (one on each side)of between 5 mm and 10 mm in diameter placed every 50 mm to 100 mm alongthe body 302 and supplied with an airflow which results in a pressuredrop through the holes of between 10 to 100 mm of water at the depth ofthe aerator 300 are suitable.

Air enters the conduit aerator 238 at an inlet 306. At the opposite endof the conduit aerator 238 is an outlet 308. The highest point on theoutlet 308 is located below the lowest point on the inlet 306 by avertical distance between the minimum and maximum expected pressure dropof water at the depth of the conduit aerotor 238 across the holes 304.The minimum expected pressure drop of water at the depth of the aerator300 across the holes 304 is preferably at least as much as the distancebetween the top of the holes 304 and the interior bottom of the body302. An air/water interface 309 between the air in the conduit aerator238 and the water surrounding the conduit aerator 238 will be locatedbelow the interior bottom of the body 302 but above the highest point onthe outlet 308. In this way, tank water 18 entering the conduit aerator238 will flow to the outlet 308 and not accumulate near the holes 304.

Now referring to FIG. 9B, another conduit aerator 238 is shown which ispreferred for use with relatively clean tank water 18. The body 302 hasa rectangular cross section but is open on the bottom. The conduitaerator 238 may be a separate component or integrated into the headers22 of a membrane module 20 in which case the bottom of a lower header 22may serve as the top of the body 302. The end of the body 302 is cappedwith a cap 310 which again may be a part of a header 22. With the bottomof the body 302 open to the tank water 18, tank water 18 which seepsinto the conduit aerator 238 flows back to the tank water 18. To preventbubbles 36 from forming at the bottom of the conduit aerator 238, thesides of the body 302 extend below the bottom of the holes 304 by adistance greater than the expected pressure drop through the holes 304.

Now referring to FIG. 9C, another conduit aerator 238 is similar to theconduit aerator 238 of FIG. 9A except as will be described herein. Arubber sleeve 400, shown partially cut away, covers the body 302 and hasslits 402 corresponding with the holes 304. The slits 402 open when airis flowed into the conduit aerator 238 opening to a larger size when ahigher rate of air flow is used. Accordingly, the slits 402 producelarger bubbles 36 at the full rate of air flow and smaller bubbles 36 atthe reduced rate of air flow. In wastewater applications, the reducedsize of the bubbles 36 provides improved oxygen transfer efficiency atthe reduced rate of air flow.

Now referring to FIG. 9D, another conduit aerator is shown which ispreferred for use with relatively solids rich tank water 18. The body302 is a tube 32 mm in diameter. The holes 304 are 8 mm in diameter andmounted 30 degrees upwards of horizontal. Drainage holes 410, at thebottom of the body 302 and typically 16 mm in diameter, allow tank water18 seepage to drain from the body 302. A cap 412 covers the end of thebody 302.

Conduit aerators 238 such as those described above may admit some tankwater 18, even with air flowing through them, which dries out leaving anaccumulation of solids. When the supply of air is switched betweenmanifolds as described above, however, the conduit aerator 238 isalternately flooded and emptied. The resulting cyclical wetting of theconduit aerators 238 helps re-wet and remove solids accumulating in theconduit aerators 238 or to prevent tank water 18 from drying anddepositing solids in the conduit aerators 238. If necessary, thisflooding can be encouraged by releasing air from the appropriatemanifold by opening a valve vented to atmosphere.

Embodiments similar to those described above can be made in manyalternate configurations and operated according to many alternatemethods within the teachings of the invention.

EXAMPLES

The following examples refer to ZW 500 membrane modules produced byZENON Environmental Inc. Each ZW 500 has two rectangular skeins ofvertical hollow fiber membranes. For the purposes of calculatingsuperficial velocities, the cross sectional area of aeration for each ZW500 membrane module is approximately 0.175 m². All air flow rates givenbelow are at standard conditions.

Example 1

A cassette of 8 ZW 500 membrane modules were operated in bentonitesuspension under generally constant process parameters but for changesin flux and aeration. A fouling rate of the membranes was monitored toassess the effectiveness of the aeration. Aeration was supplied to thecassette at constant rates of 204 m³/h (ie. 25.5 m³/h per module) and136 m³/h and according to various cycling regimes. In the cycled tests,a total air supply of 136 m³/h was cycled between aerators located belowthe modules and aerators located between and beside the modules incycles of the durations indicated in FIG. 10A. Aeration at 136 m³/h in30 second cycles (15 seconds of air to each set of aerators) wasapproximately as effective as non-cycled aeration at 204 m³/h.

Example 2

The same apparatus as described in example 1 was tested under generallyconstant process parameters but for the variations in air flow indicatedin FIG. 10B. In particular, 70% of the total air flow of 136 m³/h wascycled in a 20 second cycle such that each group of aerators received70% of the total airflow for 10 seconds and 30% of the total airflow for10 seconds. As shown in FIG. 10B, cycling 70% of the air flow resultedin reduced fouling rate at high permeate flux compared to constantaeration at the same total air flow.

Example 3

2 ZW 500 membrane modules were operated to produce drinking water from anatural supply of feed water. Operating parameters were kept constantbut for changes in aeration. The modules were first operated forapproximately 10 days with non-cycled aeration at 25.5 m³/h per module(for a total system airflow 51 m³/h). For a subsequent period of aboutthree days, air was cycled from aerators near one set of modules toaerators near another set of modules such that each module was aeratedat 12.8 m³/h for 10 seconds and then not aerated for a period of 10seconds (for a total system airflow of 12.8 m³/h). For a subsequentperiod of about 10 days, the modules were aerated such that each modulewas aerated at 25.5 m³/h for 10 seconds and then not aerated for aperiod of 10 seconds (for a total system airflow of 25.5 m³/h). For asubsequent period of about 10 days, the initial constant airflow wasrestored. As shown in FIG. 10C, with aeration such that each module wasaerated at 25.5 m³/h for 10 seconds and then not aerated for a period of10 seconds (ie. one half of the initial total system airflow), themembrane permeability stabilized at over 250 L/m²/h/bar whereas withnon-cycled airflow at the initial total system airflow the membranepermeability stabilised at only about 125 L/m²/h/bar.

Example 4

3 units each containing 2 ZW 500 membrane modules ere operated atvarious fluxes in a membrane bioreactor. Unit 1 had modules operating at26 L/m²/h and 51 L/m²/h. Unit 2 had modules operating at 31 L/m²/h and46 L/m²/h. Unit 3 had modules operating at 34 L/m²/h and 51 L/m²/h. Theunits were first operated for a period of about 10 days with non-cycledaeration at 42.5 m³/h per module (total system air flow of 85 m³/h). Thepermeability decreased and stabilized at between 250 and 275 L/m²/h/barfor Unit 1, between 200 and 225 L/m²/h/bar for Unit 2 and between 150and 175 L/m²/h/bar for Unit 3. For a second period of about 14 days, atotal system airflow of 61.2 m³/h was applied for 10 seconds to aeratorsbelow the modules and then for 10 seconds to aerators beside themodules. Under these conditions, permeability increased and stabilizedat between 350 and 375 L/m²/h/bar for Unit 1 and between 325 and 350L/m²/h/bar for Units 2 and 3.

Example 5

A cassette of 6 ZW 500 modules was used to treat sewage. While holdingother process parameters generally constant, aeration was varied andpermeability of the modules was measured periodically as shown in FIG.11. In period A, 255 m³/h of air was supplied continuously and evenly tothe modules. In period B, 184 m³/h of air was applied for 10 seconds toaerators below the modules and then for 10 seconds to aerators besidethe modules. In Period C. the same aeration regime was used, butshrouding around the modules was altered. In period D, 184 m³/h of airwas applied for 10 seconds to aerators near a first set of modules andthen for 10 seconds to aerators near a second set of modules. In periodE, 204 m³/h of air was applied to all of the modules evenly for 10seconds and then no air was supplied to the modules for 10 seconds. InPeriod F, 306 m³/h was applied to all of the modules evenly for 10seconds and then no air was supplied to the modules for 10 seconds. InPeriod G, 153 m³/h was applied to aerators near a first set of modulesand then for 10 seconds to aerators near a second set of modules.

Example 6

A single ZW 500 membrane module was used to filter a supply of surfacewater. While keeping other process parameters constant, the module wasoperated under various aeration regimes and its permeability recordedperiodically. First the module was operated with constant aeration at(a) 20.4 m³/h and (b) 25.5 m³/h. After an initial decrease inpermeability, permeability stabilised at (a) about 200 L/m²/h/bar and(b) between 275 and 300 L/m²/h/bar respectively. In a first experiment,aeration was supplied to the module at 25.5 m³/h for two minutes andthen turned off for 2 minutes. In this trial, permeability decreasedrapidly and could not be sustained at acceptable levels. In anotherexperiment, however, aeration was supplied to the module at 25.5 m³/hfor 30 seconds and then at 8.5 m³/h for 30 seconds. In this trial,permeability again decreased initially but then stabilised at between275 and 300 L/m²/h/bar.

1. A process for cleaning or inhibiting the fouling of membranesimmersed in water in a tank with outsides of the membranes in contactwith the water, the method comprising the steps of, providing a flow ofa gas which produces bubbles in the water, which bubbles rise andcontact the membranes, under an aeration regime effective to clean orinhibit fouling of the membranes, wherein the flow of gas is alternatedin repeated cycles of not more than 120 seconds in duration between ahigher flow rate and a lower flow rate, the lower flow rate being in therange between a condition in which bubbles are not produced and lessthan one half of the higher flow rate.
 2. A process according to claim 1wherein the bubbles are produced from first and second aerators, andduring a first period the second aerators receive gas at the lower flowrate while the first aerators receive gas at the higher flow rate, andduring a second period the second aerators receive gas at the higherflow rate while the first aerators receive gas at the lower flow rate.3. A process according to claim 2, wherein the second aerators areinterspersed in a horizontal plane with first aerators.
 4. The processof claim 1 wherein the membranes are hollow fiber membranes orientedvertically.
 5. The process of claim 4 wherein the membranes are between400 mm to 1,800 mm long and held at their upper and lower ends withbetween 0.1% and 5% slack.
 6. The process of claim 5 wherein the tank isbetween 1 m and 10 m deep.
 7. The process of claim 6 wherein the higherflow rate corresponds to a superficial velocity of between 0.013 m/s and0.15 m/s.
 8. The process of claim 7 wherein the flow of gas isalternated abruptly between the higher flow rate and the lower flowrate.
 9. The process of claim 8 wherein the higher flow rate is providedfor between one sixth and about one half of each cycle.
 10. The processof claim 9 wherein the higher flow rate is provided for between onethird and about one half of each cycle.
 11. The process of claim 10wherein the higher flow rate is provided for about one half of eachcycle.
 12. The process of claim 2 wherein the membranes are arrangedinto modules and the first aerators are located to supply bubbles to afirst set of modules and the second aerators are located to supplybubbles to a second set of modules.
 13. The process of claim 12 whereinthe first aerators are located in a first zone or tank and the secondaerators are located in a second zone or tank.